A nonvolatile semiconductor memory device such as a NAND type flash memory has been increased in capacity and reduced in size year after year in accordance with the progress of micro-fabrication techniques for reducing the size of memory cells of the device. It is desired for the device to further improve the yield and reduce the cost.
In a NAND process, when forming an air gap between memory cell transistors, a sacrificial layer is formed between memory cell transistors, in sidewall insulators of selection transistors, and in sidewall insulators of peripheral transistors, and then the sacrificial layer is removed, for example. An example of the sacrificial layer includes a silicon nitride (SiN) layer.
However, when removing the sacrificial layer, a part of the sacrificial layer may remain depending on the processing condition. When the sacrificial layer remains, the characteristics of the transistors are changed. In particular, in the case where the sacrificial layer is made of the SiN layer, when a high voltage such as a write or read voltage is applied, electric charges may be trapped in the SiN layer and thereby the threshold voltages of the transistors are changed to make the circuit operation difficult. Therefore, it is desirable to prevent the sacrificial layer from remaining.